A laminated glass is a safety glass because few glass fragments are scattered even if it is broken by impact from the outside. Therefore, the laminated glass has been used widely for automobiles, railway cars, aircrafts, vessels, buildings, and the like. The laminated glass has an interlayer film interposed between a pair of glass sheets.
Recently, reduction in thickness of the laminated glass has been considered for weight reduction of the laminated glass. Reduction in thickness of the laminated glass however causes deterioration in sound insulation of the laminated glass. Use of the laminated glass with poor sound insulation as windshields of automobiles may problematically result in insufficient sound insulation for sound with a frequency of about 5000 Hz such as whizzing sound and driving sound of windshield wipers.
Change in materials of interlayer films is now considered to increase the sound insulation of the laminated glass.
Patent Document 1 provides one example of the interlayer film for laminated glass; that is, Patent Document 1 teaches a sound insulation layer that contains 100 parts by weight of a polyvinyl acetal resin having an acetalization degree of 60 to 85 mol %, 0.001 to 1.0 part by weight of at least one metal salt of alkali metal salts and alkaline earth metal salts, and 30 parts by weight or more of a plasticizer. This sound insulation layer can be used alone as an interlayer film, or used as a multilayer interlayer film in the form of a stack with other layer(s).    Patent Document 1: JP 2007-070200 A